Spontaneously Self-Assembled Polar Asymmetric Multilayers Formed by Complementary H-Bonds M. S. Johal, ² Y. W. Cao, ‡ X. D. Chai, ‡ L. B. Smilowitz, ² J. M. Robinson, ² T. J. Li, ‡ D. McBranch, ² and DeQuan Li* ,² Chemical Science & Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, and Chemistry Department, Jilin University, Changchun, People’s Republic of China Received March 16, 1999 Revised Manuscript Received May 17, 1999 Ordered molecular assemblies can lead to materials with extremely high second-order nonlinear optical (NLO) properties. 1-3 Although organic molecules with high nonlinearities are well-known, 4,5 it has been dif- ficult to design bulk materials in which such molecules are well-aligned. Generating ordered assemblies by the Langmuir-Blodgett (LB) technique often leads to meta- stable films and is only possible for a narrow range of molecular species 6 while chemical synthesis of co- valently bound supramolecular structures on surfaces can be difficult and time-consuming. 7 The formation of polar multilayers by drop-casting is an attractive alter- native, especially if such structures lead to thermody- namically stable asymmetric (head-to-tail) assemblies. Asymmetric assemblies are more desirable because the inherent nonlinear optical properties of the molecular layers are additive, whereas in symmetric systems, the nonlinear response cancels within each symmetric bi- layer due to destructive interference. In this work, we describe the preparation of a novel asymmetric assembly and use sum-frequency genera- tion (ω 1 + ω 2 ) and second-harmonic generation (where ω 1 ) ω 2 ) to determine the degree of structural order and the second-order nonlinear susceptibility (d 33 ), respec- tively. Sum-frequency generation (SFG) 8 is a general second-order nonlinear optical process that probes only noncentrosymmetric regions such as interfaces. In a typical experiment, spatial and temporal overlap of visible (ω 1 ) and IR pulses (ω 2 ) at the sample gives rise to light at ω 1 + ω 2 and this signal can be used to measure either electronic spectra by varying the visible excitation (ω 1 ) or vibrational spectra by tuning the IR frequency (ω 2 ). The spontaneously self-assembled, polar multilayer films (Figure 1), which consist of a melamine and barbituric acid interlocked hydrogen bonding network, 9-12 were grown by drop-casting on a silica substrate. One of the interesting features of these materials is that they are initially formed by weak intermolecular interactions (hydrogen bonds) and yet ultimately yield thermodynamically stable and robust macroscale structures with a net polar orientation. Figure 1 outlines the synthesis of 5-[4-(dodecyloxyl)- benzylidiene]-2,4,6-(1H,3H)-pyrimidinetrione (DBP) and 2-amino-4,6-(didodecylamino)-S-triazine (ADT). DBP was synthesized by refluxing 4-dodecyloxylbenzaldehyde with barbituric acid in ethanol; ADT was obtained by treating 2-amino-4,6-dichloro-S-triazine with dodecyl- amine in DMSO with K 2 CO 3 present. DBP and ADT have complementary H-bonding codes and spontane- ously form supramolecular ribbons when mixed in a 1:1 ratio in chloroform. The formation of H-bonds in the supramolecular ribbon was confirmed by FTIR spec- troscopy as indicated by the shift of the carbonyl, amide, and amino IR bands. The resulting supramolecular ribbon assembled perpendicular to the surface. The ribbon is asymmetric because ADT has twice as many alkyl chains as DBP. Bragg diffraction (X-ray) was observed at 2θ ) 2.16, 3.18, and 4.28°, which shows that the DBP:ADT system is a multilayered lamellar struc- ture. The d spacing value obtained from X-ray diffrac- tion is 41 Å, in agreement with the ∼40 Å supramolec- ular ribbon width obtained from molecular 3D modeling. The asymmetric DBP:ADT ribbon can pack into either a symmetric structure (head-to-head and tail-to-tail) or an asymmetric structure (head-to-tail). It is found that the asymmetric DBP:ADT ribbons assemble into a polar multilayer head-to-tail structure (vide infra). It is extremely unusual to find a dipolar system that will self- assemble into a polar multilayer because dipole-dipole repulsion typically leads to energy-minimized head-to- head (or tail-to-tail) structures. The formation of these supramolecular self-assemblies is driven primarily by the interplay of encoded six H-bonds, and secondarily by substrate-film and hydrophobic chain-chain inter- actions. The films were characterized by FTIR-ATR spectros- copy, sum-frequency generation, and second-harmonic generation (SHG). The FTIR spectra were taken at 20° angle of incidence with 1 cm -1 resolution. SFG spectra were obtained by overlapping tunable mid-IR and 1064- nm beams at the sample surface. A Nd:YAG laser provided ∼35 ps pulses at 1064 nm. The energy density at the sample was ∼1 mJ/mm 2 . The mid-IR beam (100 μJ/pulse near 2.8 μm) was obtained by optical paramet- ² Los Alamos National Laboratory. ‡ Jilin University. (1) Kajiyama, T., Aizawa, M., Ed. New Developments in Construc- tions and Functions of Organic Thin Films; Elsevier Science B.V.: Amsterdam, 1996. (2) Li, D. Q.; Marks, T. J.; Zhang, C.; Wang, G. W. Synth. Met. 1991, 41-43, 3157. (3) Aldrovandi, S.; Borsa, F.; Lascialfari, A.; Tongnetii, V. J. J. Appl. Phys. 1991, 67 (9,2A), 5914. (4) Shen, Y. R. The Principles of Nonlinear Optics; John Wiley & Sons: New York, 1984. (5) Zernike, F.; Midwinter, J. E. Applied Nonlinear Optics; John Wiley & Sons: New York, 1973. (6) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir Blodgett to Self-Assembly; Academic Press Inc.: San Diego, 1991. (7) See, for example: Yang, X.; McBranch, D.; Swanson, B.; Li, D. Q. Angew. Chem., Int. Ed. Engl. 1996, 35 (5), 538. (8) For a review of SFG spectroscopy, see: Shen, Y. R. Nature 1989, 337, 519. (9) Koyano, H.; Bissel, P.; Yoshihara, K.; Ariga, K.; Kunitake, T. Langmuir 1997, 13 (20), 5426. (10) Bohanon, T. M.; Denzinger, S.; Fink, R.; Paulus, W.; Ringsdorf, H.; Weck, M. Angew. Chem., Intl. Ed. Engl. 1995, 34, 58. (11) Marchiartzner, V.; Artzner, F.; Karthaus,O.; Shimomura, M.; Ariga, K.; Kunitake, T.; Lehn, J. M. Langmuir 1998, 14, 5164. (12) Yang, W. S.; Chen, S. G.; Chai, X. D.; Cao, Y. W.; Lu, R.; Chai, W. P.; Jiang, Y. S.; Li, T. J.; Lehn, J. M. Synth. Met. 1995, 71, 2107. 1962 Chem. Mater. 1999, 11, 1962-1965 10.1021/cm990158m CCC: $18.00 © 1999 American Chemical Society Published on Web 07/14/1999